Slow & Fast-Twitch Muscle Fibers

Hey friends!

It’s going to be a beautiful day, I can just feel it. <3

Last night I full on crashed after my evening snack. It was gloooorrrious. I actually ended up showering 3 different times yesterday, and getting in some serious work at the gym. I love days like that! Endorphins are real, baby.


Did you know that your body is made up of lots & lots (and I mean LOTS) of muscle cells -aka muscle fibers? I am sure you did. But did you know that there are two main types of muscle fibers?

Let’s explore that!

Slow & Fast-Twitch Muscle Fibers

Skeletal muscle is composed of bundles of individual muscle fibers, and each of those fibers has various proteins in them that slide, grab on to, and pull on one another, causing muscle activation, movement, contractions, etc.

{VERY simplified exercise physiology. Just trying to keep it real.}


With that background of WHAT exactly we’re talking about here, let’s break it down into the two main types of fibers that our muscles are composed of. There are distinctions because each type tends to respond better to certain training protocols, exercises, and physical activities.

And knowing this mumbo jumbo can help YOU amp up your fitness game.

Just sayin’. πŸ˜‰

Slow-Twitch Muscle Fibers

These muscle fibers are also known as Type I, and they function best by using oxygen to create energy & forces necessary for long, sustained movements. These fibers also fire slower, allowing for continuous work over long periods of time.

{Any marathoners out there wish they had more of these?! Hah.}

Although we all have a bit of both types of muscle fibers, it is definitely believed (and proven in certain biopsies) that endurance racers have a greater proportion of slow-twitch muscle fibers than does the average person.

So what does this mean for YOU?

Well, most of us are born with 50/50 (about even for both types), but some people have more like 70/30 or 40/60 %, and having these different percentages tends to bring athletes to ‘their’ sports.

If you’re born with a greater percentage of slow-twitch muscle fibers, for example, you’ll tend to gravitate toward endurance sports or long distance running.

Fast-Twitch Muscle Fibers

These muscle fibers are also known as Type II, and they can actually be broken down into Type IIa and Type IIb fibers.

But first of all, fast-twitch muscle fibers are best at using anaerobic metabolic measures to create energy (i.e. without oxygen), fire quickly, and tire out quite quickly as well. These babies are responsible for allowing you to do quick plyometrics moves, cleans, sprints, and other quick, powerful movements efficiently & quickly.

Type IIa fibers are known as intermediate fast-twitch because these fibers can use aerobic or anaerobic means to generate force. They’re actually a combo of fast/slow twitch. And so these are definitely helpful for anyone who likes to do a mix of powerful moves for a bit longer time. (Think a string of burpees or sections of BodyPUMP classes).

Type IIb fibers are the fast twitch fibers most people talk about when they say ‘fast-twitch fibers’-as these are totally dependent on anaerobic metabolism to create energy. Quick, powerful, and tires out fast describe these fibers. Hello super fast & short sprints or power cleans!

Fun fact: if you look at biopsied fibers, slow-twitch are redder in color, and fast-twitch are whiter in color.

{Notice how they’re all pretty evenly distributed…}

{For more info about all of this jazz, go here}


Can You Change Fiber Type?

Most researchers do say that through training protocols, lots of hard work, and dedication, persons can ‘switch’ from one fiber to another. However, this is all very VERY not proven in all studies.

I remember in my exercise phys class studying these fibers & realizing that I was never, ever going to be a great sprinter. I was totally born with more slow-twitch than fast-twitch fibers (I can go on forever, but I am not the fastest sprinter in the bunch). BUT, I (like all of us) have both major types in my muscles.

And now I can sprint way faster than I could ‘back’ then.

Because I TRAINED that way. Because I use a mix of endurance & power in my workouts.

I am my own experiment. As are you. Don’t just believe what people and/or researchers tell you. If you want to be a better sprinter, or quicker at burpees or other power moves, then TRAIN at them.

If you want to be able to run longer & further without dying, then TRAIN that way.

We all have all those muscle fibers I just listed, so who’s to say you can’t train to get the maximum potential out of those bad boys?!

My professor did once tell me in answer to my question about ‘switching fibers’,

A lot of hard work pays off, especially since we (meaning researchers) don’t really know all the answers yet.

And there you have it.

Just get ‘er done using a mix of powerful moves & endurance sports. Hence why it’s best to cross-train, and not do the same exercises or workouts or moves over & over again with no change.

Those articles on doing a variety of things in our workouts are on to something. πŸ˜‰


Thankful Thursday

  1. I am super thankful for legs that can handle what I throw at them. πŸ˜‰
  2. I am really thankful for cottage cheese. That stuff is GOOD!
  3. I am currently grateful for throw pillows. It makes a room look so nice!

Have a GREAT day, my friends!!


Enter NYC Trip Sweepstakes!

Enter FitMixer Giveaway!

What type of muscles fibers do YOU feel you have a greater percentage of?? Are YOU currently trying to ‘switch up’ some of those fibers?? What are YOU thankful for today??

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  1. Hmmm I’m not sure which one I have more of.. I feel like I used to be super speedy and now I’m more endurance… Since incorporating more speed work in my half marathon training, I do feel like I’m training both which is great! :)

  2. I feel like I probably have more fast-twitch fibers…I remember doing well in the sprinting events in track and field in elementary school, but long distance running is a lot more difficult for me. Just means you’re a lot more likely to see me doing intervals than going for a 10 mile run. :-)

  3. That was super informative :). I definitely have more slow twitch fibers which is why I’m an endurance runner, but I can tell classes like Body Pump and doing plyo and some speedwork are helping with my fast twitch muscles, you are so right when you say you can train your muscles because you can :)

  4. I would assume I have more slow twitch fibers, but I guess I don’t really know. Long distance running has always appealed to me more so I’ve just always gravitated toward the idea of it. Anytime I’ve ever sprinted, I’ve always been around people who were way better than me. It made me just assume I was slow, I guess? I’m definitely much faster than I used to be after doing a lot of HIIT runs. During my runs, I would much rather run more miles than try to run fast for a few. Maybe that means I have more slow twitch fibers. Hmmm.

    • It is hard to really KNOW unless you have biopsies of your muscles taken….and those hurt πŸ˜‰ But yes, people tend to gravitate towards the sports they’re better at. I.e. why sprinters are good sprinters :) have a great day!!!! <3

  5. Haha I definitely think I have more slow twitch! I can go on forever but I am awful at sprinting or moves that require a burst of energy!

  6. I think I have more slow twitch fibers. Mostly because I was a competitive swimmer for years and trained for up to 3 hours a day. So I’m assuming from all that training I developed those, although now I like sprints more. Very interesting information! I love this science-y stuff!

  7. great info Annette! CrossFit definitely uses both fast & slow twitch fibers and I wish I had MORE of BOTH! πŸ˜‰

  8. AWESOME post lady! I definitely make sure my workouts incorporate rockin’ BOTH fast & slow twitch muscle fibers. We both know how important it is to switch things up and keep your body working! And yes, endorphins are SO. real. YOU ROCK LADY!

  9. Oh, good post…so informative! Ya, I think I am one of those 50/50 gals…just steady eddie trying to do my best to be healthier/more active/and if I get more muscle and lose some fat along the way then KUDOS to me! πŸ˜‰ However, I am mixing up the workouts to try to active more fibers I guess. πŸ˜‰

  10. I think I have the fast twitch! But then again I can go on and on in class with great endurance- so I’ve got some of those slow twitch too πŸ˜‰ I am thankful for a great sleep :)

  11. Another great great post. It’s so important to train all areas and realms to work the entire musculoskeletal system. I definitely love a mix of endurance and more power focused workouts. I think I have a pretty even mix.

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    Slow & Fast-Twitch Muscle Fibers | Type I and Type II | Training Protocols | Endurance | Gratitude | Enjoy Your Healthy Life



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